The present invention is concerned generally with fuel combustors suitable for use with gas turbines, and more particularly relates to a fuel combustor with improved cooling components for cooling the combustion chamber wall, and furthermore relates to a fuel combustor capable of alternatively and selectively burning liquid or gaseous fuels including, low heat content gaseous fuels.
Prior art combustors have not been designed to satisfactorily burn, both safely and stably, gaseous fuels of low heat content values, such as those gaseous fuels available from shale oil extraction processes. During shale oil extraction, a process offgas is yielded which is of a low heat content, but is still attractive enough for burning in gas turbine power plants especially for the generation of on and off site electrical energy.
It has been found that due to the low heat content of process gas, the amount of air required for combustors is not large enough to cool and maintain safe wall temperatures at the combustor's respective primary and secondary combustion zone enclosures by conventional cooling methods.
Various combustors for use with gas turbine systems are disclosed in U.S. Pat. Nos. 3,738,106, 3,720,497; 3,608,309; and 3,589,128. In the combustors disclosed in these patents, compressed air is passed about the combustion chamber in a direction counter to the direction of flow of the combustion gases therein, primarily for the purpose of cooling the inner liner or wall about the combustion zone before the air is delivered for mixture with the fuel to be delivered and burned in the combustion chamber. This type of counter flow operation, provides film cooling of the combustion zone wall, but cannot meet the cooling requirements necessary to burn low heat content gaseous fuels.
One of the problems encountered in the development of a combustor to burn low heat content gas was the inability of lean, low heat content gas-air mixtures to ignite and burn stably in the combustor during start-up. However, once the start-up phase has passed, combustion of the low heat content gas-air mixtures proceed satisfactorily to provide good gas turbine performance.
To overcome the start-up phase ignition problems, use of another fuel has been considered. While the volume of liquid fuel is drastically lower compared to the gaseous fuel, and its heat content per mass unit is higher, nonetheless a liquid fuel was found to be compatible with, not only the start-up phase ignition, but also with a switchover function to low heat content gaseous fuel. Use of the liquid fuel minimizes the danger of combustor flame-out or prohibitive instability which may occur when attempting start-up with the low heating content gaseous fuel. Accordingly, a combustor having a multi fuel operating capability will provide enhanced performance because at normal load conditions, it will be capable of burning low heat content gases.
A combustor for use with a gas turbine is disclosed in U.S. Pat. No. 2,648,950 which may burn a gaseous fuel injected into the primary combustion zone of the chamber and also a pulverized liquid fuel injected into the secondary combustion zone of the combustion chamber after the evaporation of the water component thereof is completed. These different fuels are injected into the combustion chamber, not only in different zones thereof, but oriented so that the nozzles oppose each other. This type of structural arrangement however does not meet or overcome the problems of ignition start-up using a low heat content gaseous fuel. Therefore, reliance upon the apparatus disclosed in this patent could not provide the particular structural arrangement which would allow the alternate burning, and selective switching, of gaseous and liquid fuels.
With the need for new sources of energy; the development of more feasible techniques for economically utilizing heretofore uneconomical forms of energy such as low heat content gases produced in shale oil extraction processes has become increasingly more attractive providing a suitable apparatus can satisfactorily burn this type of gas.
To achieve such a satisfactory combustor, the parameters desired include, the employment of alternate fuel burning capabilities to ensure good start-up performance; convenient switchover; providing controls to switchover from the liquid fuel burning mode at start-up to the gaseous fuel burning mode at normal load operations including, providing the proper amounts of air for optimum fuel mixture burning; minimizing movable or operable parts to provide the switchover ability; providing the necessary cooling requirements for both liquid burning fuel start-up and gaseous burning fuel normal mode especially for keeping the wall temperatures of the combustors primary and secondary zones within safe limits.
The present invention provides an improved combustor which can satisfy these requirements.